Copy/Pasted From:
https://www.dervosvalve.com/news/what-does-the-valve-arrow-mean-on-the-valve-body.html#:~:text=The%20arrow%20direction%20marked%20on,leakage%20and%20even%20pipeline%20accidents.

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The arrow marked on the valve body is the recommended pressure bearing direction of the valve, not the flow direction of the pipeline medium. Valves with two-way sealing function may not be marked with an indicating arrow. Sometimes they may be marked with an arrow, because this valve arrow refers to the recommended pressure bearing direction, as one of the left and right or up and down directions is always better.The arrow direction marked on the valve body indicates the pressure bearing direction of the valve, which is usually incorrectly marked as the medium flow direction that leads to false installation by engineers, causing leakage and even pipeline accidents.The pressure bearing direction refers to the recommended arrow direction mark of the valve body in the closed state after the valve is applied to the pipeline working condition. If the valve is installed incorrectly, the leakage fault of the valve may occur. Supersoft sealing ball valves are usually sealed in two directions, and no arrows are usually marked. Metal hard sealing ball valves can be sealed in two directions, but the sealing performance in one direction is still better, so arrows are also marked, which refers to the recommended pressure bearing direction of the valve. You can also consult the customer at first.Hard seal butterfly valve with marked arrow is in different positions of pipeline, and its direction and medium flow direction are different. For example, at the outlet end of pump in pump house, the arrow of valve body and medium flow direction are opposite. For example, at the inlet end of pump, the arrow and medium flow direction are consistent; if installed on main pipeline, the arrow direction generally conforms to medium flow direction, which depends on working conditions and installation position.

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From:
https://www.plasticpipevalve.com/news/how-does-the-flow-direction-through-a-globe-valve-affect-its-operation-and-efficiency.html#:~:text=In%20over%2Dseat%20flow%20configurations%2C%20the%20pressure%20differentials%20across%20the,pressure%20differentials%20and%20fluid%20velocities.

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How does the flow direction through a globe valve affect its operation and efficiency?

Posted by Admin | 10 Jun

The flow direction through a globe valve can significantly impact its operation and efficiency in several ways:

Pressure Drop: The direction of flow through a globe valve is a critical factor affecting the pressure drop experienced across the valve. Pressure drop, often measured in terms of head loss, refers to the decrease in pressure of the fluid as it passes through the valve. When fluid enters the valve from below the valve seat (under-seat flow), it encounters less obstruction, resulting in a lower pressure drop compared to when the fluid enters from above the seat (over-seat flow). This distinction is crucial because higher pressure drops require additional energy to overcome, impacting the overall efficiency of the system.

Sealing Performance: The sealing performance of a globe valve is influenced by the direction of flow. In over-seat flow configurations, the fluid pressure assists in pressing the valve disk firmly against the seat, enhancing the seal integrity and preventing leakage. This configuration is particularly advantageous in high-pressure applications where maintaining a tight seal is crucial for operational safety and efficiency. However, in under-seat flow scenarios, the fluid tends to lift the disk away from the seat, potentially compromising sealing effectiveness, especially in low-pressure conditions.

Cavitation and Erosion: Flow direction significantly impacts the likelihood of cavitation and erosion within the globe valve. Cavitation occurs when the pressure of the fluid drops below its vapor pressure, bring about the formation and collapse of vapor bubbles. In over-seat flow configurations, the pressure differentials across the valve are minimized, reducing the risk of cavitation and its detrimental effects on valve components. Conversely, under-seat flow conditions may increase the risk of cavitation and erosion due to higher pressure differentials and fluid velocities. Cavitation and erosion can cause significant damage to valve internals, bring about decreased performance, increased maintenance costs, and potential safety hazards.

Control and Stability: Flow direction plays a crucial role in determining the control and stability characteristics of the globe valve. In applications requiring precise flow regulation or modulation, such as in process industries, the flow direction can affect the valve's ability to accurately control the flow rate. Under-seat flow configurations may offer ascendant control capabilities due to enhanced fluid throttling properties, allowing for precise adjustment of flow rates to meet process requirements. However, over-seat flow configurations generally provide greater stability and resistance to pressure and flow rate fluctuations, ensuring consistent performance and operational reliability in dynamic operating conditions.

Noise and Vibration: The direction of flow through the globe valve has implications for noise and vibration levels during operation. Turbulent flow and fluid velocity fluctuations can generate noise and vibration, potentially bring about operational issues and discomfort for operators. In over-seat flow configurations, where fluid passage is smoother and more streamlined, turbulence and pressure fluctuations are minimized, resulting in reduced noise and vibration levels. Conversely, under-seat flow configurations may experience increased turbulence and fluctuations, bring about elevated noise levels and vibration. Excessive noise and vibration can impact system performance, reliability, and personnel safety.

UPVC Flange Type Globe Valve DN15-150

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From:
https://blog.craneengineering.net/valves-flow-to-open-fto-vs-flow-to-close-ftc

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Valves: Flow to Open (FTO) vs. Flow to Close (FTC)

One of the characteristics of a linear globe and rotary control valve is flow direction on the plug; flow to open (FTO) and flow to close (FTC). In this post, you'll gain a better understanding of these concepts and which is best for your application. Differences between flow to open and flow to close are explained through two simple analogies.  

Flow to Open

Also referred to as standard or forward flow, or in globe valves, flow under the seat. Think about when you've controlled the flow of water from a garden hose with your thumb. Your thumb acts like the face of the plug in a globe valve - flow is pushing against your thumb to open a flow path for the water.

Flow to Close

Also referred to as reverse flow or in globe valves, flow over the seat. Consider a drain plug in a bathtub. The flow direction is against the back or top of the plug (rather than the face) creating a tendency of the plug to close into the drain.

The following drawings illustrate flow direction for the two basic valve designs:

Linear:

Rotary:

Applications

Many variables determine which flow direction is appropriate for an application. The style of valve trim, valve (rotary or linear), and the design of the valve all determine flow direction.

Flow To Open:

Most general service applications are flow to open unless there's a reason to go to flow to close. In rotary valves without a retained seat design, having the flow direction towards the face of the plug assists the seat in sealing against the plug, resulting in tighter shutoff. In globe valves anti-cavitation and low noise trim could be either under or over the seat. Flow to open is generally best for control in low flow applications.

Flow To Close:

When anti-cavitation trim is required in a rotary valve, flow to close is used so the flow can be diffused rather than flowing into the face of the plug. Balanced trim in a general service globe valve is typically flow to closed. This is normally used in high pressure and/or throttling applications to stabilize the stem. A potential disadvantage of flow to close is reduced flow capacity.

Exception:

Below is an exception to a classic style flow to open globe valve. The flow direction is NOT against the face of the plug, but behind the plug. This is not flow to close since the plug is located beneath the seat ring. Flow direction is still considered ‘under the seat’ which is causing the plug to open.

What's often confused is that flow to open and flow to close are independent of fail open (air to close) and fail close (air to open) on an air-to-spring diaphragm actuator. The actuator set up will determine whether the spring set will open or close the valve upon removal of air supply. This discussion merely involves flow direction through the valve in relation to the valve plug.

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